1. Field of the Invention
The present invention relates generally to apparatus for mixing audio signals, and particularly to apparatus for mixing digitalized audio signals.
2. Description of the Prior Art
There has hitherto been proposed a digital signal mixing apparatus as shown in FIG. 1, in which S-channels (where S is an integer) of digitalized audio signals are mixed at a desired mixing ratio to produce new digitalized audio signals in T channels (where T is also an integer).
In FIG. 1, S digital input channels are respectively supplied through input terminals 1 to digital-to-analog (D-A) converters 2 to produce plural-channel analog input signals. Other channels of analog signal, such as microphone signals, are supplied from respective input terminals 3 to amplifiers 4. Outputs of respective D-A converters 2 and amplifiers 4 are selectively changed over by respective change-over switches 5 and fed to corresponding analog tone control circuits 6. Outputs of these tone control circuits 6 are then applied to an analog signal mixing circuit 7. Mixed outputs of the analog signal mixing circuit 7 are partially fed to an analog echo adding device 11 and its outputs are supplied through switches 5' and further through other tone control circuits 6 to the analog signal mixing circuit 7. T-channels of analog output signals from the mixing circuit 7 are respectively supplied to analog-to-digital (A-D) converters 8 to obtain a plurality of digital output signals at respective output terminals 9. At output terminals 10 output analog signals can be directly obtained.
However, the above digital signal mixing apparatus has both following defects. Since the D-A converters and A-D converters are used in this apparatus, an undesirable amount quantization noise occurs in the outputs produced at terminals 9. Further, since the mixing is carried out in analog form, a distortion occurs based upon the non-linearity of the input-output characteristics of the analog signal mixing circuit. In addition, the analog signal mixing circuit is apt to be affected by external noise and hence such external noise is also mixed into its outputs.